The present invention relates to an image forming apparatus such as a copying machine, a facsimile machine, a printer, or the like, which employs an electrophotographic or electrostatic recording method. In particular, it relates to an image forming apparatus of an indirect transfer type, in which an image made up of developer on an image bearing member is transferred onto an intermediary transfer member.
As for a typical electrophotographic image forming apparatus, an image forming apparatus such as the one shown in FIG. 10 has been known.
The image forming apparatus 200 in FIG. 10 is provided with an electrophotographic photoconductive member, in the form of a drum, that is, a photoconductive drum 1, as an image bearing member. The photoconductive member 1 is rotationally supported and is rotationally driven by a driving means (unshown) in the direction indicated by an arrow mark in the drawing. Surrounding the photoconductive drum 1 are a charging means 2, an exposing means 3, a developing means 4, and a cleaning means 10. Further, placed next to the photoconductive drum 1 are an intermediary transfer belt 6 as an intermediary transferring member, and a primary transferring means 5 which is positioned in a manner to sandwich the intermediary transfer belt 6 between itself and photoconductive drum 1.
The photoconductive drum 1 comprises an aluminum cylinder, and a layer of organic photoconductor (OPC) coated on the peripheral surface of the aluminum cylinder. The charge roller 2 comprises a metallic core, an electrically conductive rubber layer, and a surface layer, listing from the inward side. The electrical resistance of the surface layer is in the mid range. The charge roller 2 is placed in contact with the peripheral surface of the photoconductive drum 1. The peripheral surface of the photoconductive drum 1 is uniformly charged by applying to the charge roller 2 a bias (charge bias), which is a combination of DC and AC biases. The exposing means 3 emits a beam of laser light in response to signals in accordance with the image formation data inputted into a laser driver, exposing the uniformly charged peripheral surface of the photoconductive drum 1. As a result, an electrostatic latent image is formed on the peripheral surface of the photoconductive drum 1.
The developing means 4 is a rotary developing means comprising developing devices 4Y, 4M, 4C, and 4K for developing the electrostatic latent images into toner images of yellow (Y), magenta (M), cyan (C), and black (B), respectively. When developing the electrostatic latent images into toner images different in color, the rotary developing means 4 is rotated so that the developing devices different in the color of the developer contained therein are moved one by one to the location, at which developing devices oppose the photoconductive drum 1. As a result, the developers different in color are adhered to the peripheral surface of the corresponding photoconductive drum 1 in a manner to reflect the pattern of the corresponding electrostatic latent images on the peripheral surface of the photoconductive drum 1, forming images made up of the developers (toner images).
The intermediary transfer belt 6 is stretched around four rollers: a driving roller 61, a tension roller 62, a follower roller 63, and a primary transfer roller 5, and is moved (rotationally driven) in the direction indicated by an arrow mark in the drawing. Each of the toner image, which are formed on the photoconductive drum 1 and are different in color, is sequentially transferred onto the intermediary transfer belt 6 by applying the primary transfer electric field to the primary transfer roller 5 as the primary transferring means disposed at a location at which the intermediary transfer belt 6 is pinched between the photoconductive drum 1 and primary transfer roller 5. There is also the secondary transfer roller 8 as the secondary transferring means, which is disposed at a location where it opposes the follower roller 63, with the intermediary transfer belt 6 pinched between the secondary transfer roller 8 and follower roller 63. The toner images placed in layers on the intermediary transfer belt 6 are transferred all at once by applying the secondary transfer electric field to the secondary transfer roller 8, onto a recording medium P.
The cleaning means 10 for the photoconductive drum 1 is disposed downstream from the primary transfer station T, in terms of the direction in which the photoconductive drum 1 is driven. It removes the transfer residual toner particles, that is, the toner particles which were not transferred onto the intermediary transfer belt 6 in the primary transfer station T and remained on the photoconductive drum 1.
The cleaning means 11 for the intermediary transfer belt 6 is disposed on the downstream side of the secondary transfer station Txe2x80x2, in terms of the direction in which the intermediary transfer belt 6 is driven. It removes the residual toner particles, that is, the toner particles which were not transferred onto a recording medium P in the secondary transfer station Txe2x80x2 and remained on the intermediary transfer belt 6.
The fixing means 9 (fixing device) has two rollers: a fixing roller 91 and a pressure roller 92. It fixes the toner images to the recording medium P after they are transferred onto the recording medium P.
Next, the operation of the image forming apparatus 200 structured as described above will be described. First, the peripheral surface of the photoconductive drum 1 is uniformly charged by applying charge bias to the charge roller 2 while rotationally driving the photoconductive drum 1 in the direction indicated by the arrow mark in the drawing. Next, the photoconductive drum 1 is exposed by the exposing means 3 in accordance with the image formation data of the first color component, for example, yellow (Y) color component, forming an electrostatic latent image corresponding to the yellow component of an intended image. At this point of the image forming operation, the developing device rotary 4 is rotated to move the yellow component developing device 4Y to the position at which the yellow component developing device 4Y, which contains yellow toner as developer, opposes the photoconductive drum 1. Then, a compound bias (development bias), which normally is a combination of DC and AC voltages, is applied to the developer bearing member (development roller) of the yellow component developing device 4Y to adhere the yellow toner to the electrostatic image on the photoconductive drum 1. As a result, a toner image is made up of yellow toner, on the peripheral surface of the photoconductive drum 1. Then, the yellow toner image on the photoconductive drum 1 is transferred onto the intermediary transfer belt 6 by applying the primary transfer bias to the primary transfer roller 5.
Similarly, the toner images corresponding to magenta (M), cyan (C), and black (K) color components, respectively, are sequentially formed on the photoconductive drum 1, and are transferred in layers onto the intermediary transfer belt 6. As a result, four toner images different in color are placed in layers on the intermediary transfer belt 6.
Meanwhile, the recording medium P stored in a cassette 12 as a recording medium storing portion are consecutively supplied into the image forming apparatus 200 while being separated from the other recording medium P. Each recording medium P is conveyed to the secondary transfer station Txe2x80x2 by a conveyance roller pair 14 and a registration roller pair 15 in synchronism with the movement of the four color toner images on the intermediary transfer belt 6.
In the secondary transfer station Txe2x80x2, the four color toner images layered on the intermediary transfer belt 6 are transferred all at once, by applying secondary transfer bias to the secondary transfer roller 8, onto the recording medium P which is being fed into the secondary transfer station Txe2x80x2 with a predetermined timing.
After the transfer, the toner images on the recording medium P are fixed by the fixing device 9; toner images on the recording medium P are turned into a permanent full-color image. Thereafter, the recording medium P is discharged from the image forming apparatus by a combination of a conveyance roller pair 16, a discharge roller pair 17, and the like.
Prior to the employment of an intermediary transfer type image formation process such as the above described one, a multi-transfer type image formation process had been employed, in which a plurality of toner images were sequentially transferred in layers onto a recording medium kept electrostatically adhered to a recording medium conveying means such as a conveyer belt. In the case of this type of transferring method in which a plurality of toner images are transferred in layers onto a recording medium, image quality is largely dependent upon recording medium properties, for example, the size, thickness, and surface roughness, as well as recording medium uniformity which is affected by the presence of gaps in the recording medium.
In comparison, according to the intermediary transfer type image formation process, the toner images are layered on an intermediary transferring member, the base layer of which is formed of resin, that is, material with uniform consistency. Therefore, the problem that image quality is affected by the recording medium properties can be avoided to improve image quality.
As for the intermediary transferring member, there are two essential types: a drum type and a belt type. In consideration of the reduction of image forming apparatus size and spacial efficiency, the selection of the belt type intermediary transfer member is preferable, since it affords more latitude in the mechanical design of an image forming apparatus.
However, in the case of an intermediary transfer type image formation process which employs an intermediary transfer belt, the material for the intermediary transfer belt onto which toner images are transferred in the primary transfer process is required to be uniform in consistency.
For example, the fact that the surface of an intermediary transfer belt is rough means that there are a large number of high peaks and low valleys on the surface of the intermediary transfer belt as shown in FIG. 11(a). With the presence of such peaks and valleys, it is possible that the photoconductive drum and primary transfer roller pinch the intermediary transfer belt as shown in FIG. 11(b); in other words, the intermediary transfer belt fails to uniformly contact the photoconductive drum and transfer roller, creating an unsatisfactory transfer electric field.
As an unsatisfactory transfer electric field is created, the efficiency with which image forming dots are transferred reduces. For example, in the area of the intermediary transfer belt, shown in FIG. 12, across which an unsatisfactory transfer electrical field is created, the dot transfer efficiency is lower. If the location of such an area coincides with a given portion of the solid portion of of an image, it is possible that this portion will be developed into an area (hatched portion), shown in FIG. 13(a), the density of which is lower than the intended density, that is, the density of the surrounding area. Further, if the location of such an area coincides a given halftone portion of an image, when forming the given halftone portion of an image, with the use of small dots in accordance with the gradation method based on dot area ratio, some of the small dots will not be transferred at all, resulting in an image defect, that is, an unintended white spot, as shown in FIG. 13(b).
The present invention was made in consideration of the above described issues, and its primary object is to provide an image forming apparatus, in which the intermediary transferring member uniformly contacts both the image bearing member and transferring member, preventing therefore the occurrence of image defects traceable to the surface configuration of the intermediary transferring member, so that high quality images can be formed.
A preferable embodiment of an image forming apparatus for accomplishing the above object comprises:
an image bearing member for bearing an image made up of developer;
an image forming means for forming a developer image, on said image bearing member;
an intermediary transferring member onto which the developer image on said image bearing member is transferred;
a primary transferring member, which is disposed at a position where it opposes said image bearing member, pressing said intermediary transferring member upon said image bearing member, with said intermediary transferring member interposed between the said primary transferring member and image bearing member, and transfers the developer image on said image bearing member onto said intermediary transferring member as voltage is applied to the primary transferring member; and
a secondary transferring member for transferring the developer image on said intermediary transferring member onto a recording medium,
wherein the following relation is satisfied:
xcex8a less than 2.355 xc3x97(P+11.4)/(R+94.2)xe2x88x920.0174xc3x97Rmax 
in which
Rmax (xcexcm) is the maximum value of the height of the projection, which represents the surface roughness of the said intermediary transferring member onto which a developer image is transferred;
xcex8a (*) is an average inclination angle which also represents the surface roughness of the said intermediary transferring member onto which a developer image is transferred;
P (N) is the amount of the pressure applied to keep said transferring member pressed upon said intermediary transferring member; and
R (*) is the hardness, in Asker C scale, of said transferring member.
These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings.